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AbstractAbstract
[en] The glass used to store high-level radioactive waste is produced by reaction of a solid waste residue and a glassy precursor (glass frit). The waste residue is first dried and calcined (to lose water and nitrogen respectively), then mixed with the glass frit to enable vitrification at high temperature. In order to obtain a good quality glass of constant composition upon cooling, the chemical reactions between the solid precursors must be complete while in the liquid state, to enable incorporation of the radioactive elements into the glassy matrix. The physical and chemical conditions during glass synthesis (e.g. temperature, relative proportions of frit and calcine, amount of radioactive charge) are typically empirically adjusted to obtain a satisfactory final product. The aim of this work is to provide new insights into the chemical and physical interactions that take place during vitrification and to provide data for a mathematical model that has been developed to simulate the chemical reactions. The consequences of the different chemical reactions that involve solid, liquid and gaseous phases are described (thermal effects, changes in crystal morphology and composition, variations in melt properties and structure). In a first series of experiments, a simplified analogue of the calcine (NaNO3-Al2O3 ± MoO3/Nd2O3) has been studied. In a second series of experiments, the simplified calcines have been reacted with a simplified glass frit (SiO2-Na2O-B2O3-Al2O3) at high temperature. The results show that crystallization of the calcine may take place before interaction with the glass frit, but that the reactivity with the glass at high temperature is a function of the nature and stoichiometry of the crystalline phases which form at low temperature. The results also highlight how the mixing of the starting materials, the physical properties of the frit (viscosity, glass transition temperature) and the Na2O/Al2O3 of the calcine but also its crystallization all contribute to the success of the vitrification process and the homogeneity of the final glass. (author)
[fr]
Le verre de conditionnement des dechets issus des solutions radioactives de haute activite est le produit des reactions chimiques entre un precurseur vitreux (fritte de verre) et le dechet. Par une succession d'etape, le dechet est seche, denitre (calcination) puis melange a la fritte de verre (vitrification) pour etre vitrifie a haute temperature. Afin d'obtenir un verre monophase a l'etat liquide puis un verre de qualite optimale apres refroidissement, les reactions doivent etre completes pour permettre l'incorporation totale du dechet au sein du reseau vitreux. Dans cet objectif, les parametres physico-chimiques mis en jeu lors de l'etape de calcination et de vitrification sont fixes grace a des regles empiriques (temperature, taux d'adjuvant, taux de charge en dechet). Complementaire a cette approche, ce travail apporte de nouveaux elements de comprehension de la chimie et des mecanismes reactionnels mis en jeu dans le processus de vitrification, et permet de developper un modele mathematique simulant les reactions chimiques dans la zone reactive lors de l'elaboration du verre de dechet. Les resultats experimentaux obtenus permettent de decrire precisement les differentes reactions chimiques en temperature a l'etat solide, liquide et gazeux (thermique, morphologique, microstructurale et structurale), de calcinats simplifies (NaNO3-Al2O3 ± MoO3/Nd2O3) en l'absence et en presence de fritte de verre (SiO2-Na2O-B2O3-Al2O3). Ils demontrent en particulier que la cristallisation du calcinat en elle-meme n'est pas nefaste a la reactivite chimique mais que la nature et la stoechiometrie des phases cristallines formees ainsi que les cinetiques de transformations des calcinats (denitration, cristallisation) peuvent modifier la reactivite chimique avec le verre et l'avancement des reactions. Ces resultats mettent alors en evidence comment le mode de dispersion des precurseurs, les proprietes physiques de la fritte de verre (viscosite, transition vitreuse) et la composition du calcinat (rapport Al2O3/Na2O) mais aussi sa cristallisation, peuvent modifier le processus de vitrification et l'homogeneite du verre finalOriginal Title
Etude des mecanismes de reactivite chimique des precurseurs lors de l'elaboration d'un verre de confinement de dechet de haute activite. De l'experimentation a la modelisation
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27 Sep 2012; 285 p; 164 refs.; Available from the INIS Liaison Officer for France, see the 'INIS contacts' section of the INIS-NKM website for current contact and E-mail addresses: http://www.iaea.org/INIS/contacts/; Sciences de la Terre et de l'Environnement
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